Chromium, molybdenum ferritic stainless steels

ABSTRACT

A ferritic alloy containing, in general ranges, 27-32.50% chromium, 1.8-5.8% molybdenum, 0.25-3.0 nickel, 100 ppm carbon maximum, 200 ppm nitrogen maximum, the sum of carbon plus nitrogen being 250 ppm maximum, having inherent post-welding ductility and high corrosion resistance.

United States Patent Streicher Dec. 30, 1975 CHROMIUM, MOLYBDENUMFERRITIC [56] References Cited STAINLESS STEELS UNITED STATES PATENTS[75] Inventor: Michael A. Streicher, Wilmington, 2,183,715 12/1939Franks 75/126 F Del. 2,274,999 3/1942 Allen 75/126 C v 2,624,671 1/1953Binder.... 75/126 C Asslgneelde Nemours & 3,672,876 6/1972 Sipos 75 124I Company, Wilmington, Del. [22] Ffl d; M 30, 1974 Primary ExaminerL.Dewayne Rutledge pp NOI: 474,543 Assistant ExaminerArthur J. SteinerRelated US. Application Data [57] ABSTRACT- Division March 1971, Aferritic alloy containing, in general ranges, abandoned, which is acontinuation-impart of Ser. 27 32 5 Chromium, 1 g 5 g% l bd 46,428 l1970 abandoned- O.253.0 nickel, 100 ppm carbon maximum, 200 ppm nitrogenmaximum, the sum of carbon plus nitrogen [52] US. Cl; /128 W; 75/128 Nbeing 2 0 pp maximum having inherent p [51] Int. Cl. C22C 38/44 weldingductility and high corrosion resistance [58] Field of Search 75/128 W,128 N 2 Claims, 2 Drawing Figures m l a "50' b PP" E 6 660 25521 1 Maelflflpprw l T mill/z 200 1rb r 04 PP 5 \w 2 M Fame mag/mama 5-0 2 an51.9 I m we g 404: r 41 I0 .9 .418 521 512 53%1 Q 650 29 a 50 571 A .21729 m w 508* 419 0 e9 & 9I" z 67 E I I .957 um l l i l 1 l I 1- 22' 24 262a 50 32 34 Per cent qy vt eyfltabro Passed W -6'L TestbutFazYedBeadTesb Failed R961 50110 41m 22m FazMRCIg mum dikzl rem +aemzmay US. Patent Dec. 30, 1975 Sheet 1 of2 3,929,473

w mw

Perlezzi 1 29 WeyZM ybdenw/L US. Patent Dec. 30, 1975 Sheet 2 of23,929,473

CHROMIUM, MOLYBDENUM FERRITIC STAINLESS STEELS CROSS REFERENCE TORELATED APPLICATION This is a division of application Ser. No. 122,529,filed Mar. 9, 1971, now abandoned which in turn is acontinuation-in-part of U.S. Pat. application Ser.-No. 46,428 filed June15, 1970, now abandoned.

BRIEF SUMMARY OF THE INVENTION Generally, this invention comprises acorrosion-, resistant especially pitting-resistant ferritic alloy havinggood post-welding ductility containing, as principal alloying element,chromium and molybdenum in the combinations lying within areas A A B, CC and D of FIG. 1 of this application, carbon 100 ppm maximum, nitrogen200 ppm maximum, and carbon plus nitrogen 250 ppm maximum, the remainderbeing iron and incidental impurities.

The essential components of the alloys of this invention are Fe, Cr, Moand certain metal additives hereinafter identified. As in all alloys ofthe class involved, there may also be present incidental impurities. Incommercial practice these might consist of the following, in theapproximate weight percentages reported:v S, 0.010%, P, 0.010% (togetherwith, typically, 0.80% Mn and 0.50% Si as deliberate additions).

DRAWINGS The following drawings present the essential requirements interms of percent chromium as abscissa and percent molybdenum as ordinatetogether with the permissible carbon and nitrogen contents requiredaccording to this invention, in which:

FIG. 1 is a plot of four different regions of different corrosionresistance and post-weld ductility for alloys containing C equal to orbelow 100 ppm, N equal to or below 200 ppm, and C+N equal to or below250 ppm, and

FIG. 2 is an overlay of the same regions of corrosion resistance andpost-weld ductility as FIG. 1 within which are plotted typical ferriticCr, Mo alloy compositions matching those of FIG. 1, except that the Ccontent is above 100 ppm, or the N content is above 200 ppm, or C+N isabove 250 ppm.

In the early development of the stainless steels, chromium steelscontaining 12-14% Cr and 14% C were the first, large-volume products.Attempts were soon made (Br. Pat. No. 18,212 accepted on July 9, 1914)to improve the corrosion resistance properties by the addition ofmolybdenum; however, it was noted that molybdenum, when applied insufficient quantity to make the alloy passive, also made it too hard andbrittle. Brittleness contributed by Mo addition was confirmed by Reitzet al. in U.S. Pat. Nos. 2,110,891 and 2,207,554. Franks U.S. Pat. No.2,183,715 taught additions of 1-5% of Mo to iron, chromium alloys butfound this addition insufficient to overcome even his mild serviceexposures and recommended the addition of niobium to the extent of fourtimes the carbon content, at least, to overcome his problems of pittingcorrosion.

Finally, Moneypenny, in Stainlesslr on and Steel, Vol.

1, Chapman & Hall, London, 1947, p. 48, reported certain contemporaneouswork done in Germany to improve the usefulness of iron chrcmium alloysby adding about 2% Mo to them. While resistance to corrosion by a numberof organic acids and other'com- 2 pounds was reported-to be markedlyincreased, especially at Cr contents above about 18%, the mechanicalproperties were'not improved. Thus, the alloys were classed asnotch-brittle and subject to marked grain growth when heated to hightemperatures, as, for example, during welding.

It has been generally recognized, up to this date, that Fe, Cralloysas aclass develop a high degree of brittleness in or adjacent to welds, andthis inadequacy has severely limited uses of the alloys containing morethan about 20% Cr wherever welding is essential as, for example, in themanufacture of chemical processing and other vessels, pipes and similarequipment.

Early investigators were able to reduce the impact brittleness offerritic chromium alloys by limiting combined carbon and nitrogencontents to about 0.023% maximum, as reported in U.S. Pat. No.2,624,671; however, marked post-welding brittleness persisted and, inU.S. Pat. No. 2,624,670, it was reported necessary to convert the alloysto at least a pa'rtially austenitic state in order to cure thedifficulty. But austenitic alloys are subject to chloridestress-corrosion cracking, and so one valuable attribute was lost in theacquisition .of another. Moreover, these investigators deemed itnecessary to heat treat by annealing at 900C., followed by rapidquenching, in order to minimize brittleness in weldments, and this is anexceedingly troublesome and expensive expedient.

Corrosion is anextremely complex combination of phenomena constitutingnumerous well-recognized types. To detect and overcome susceptibility tothe individual types of corrosionrequires individually designedtechniques for each. It is also not generally true that a materialresistant to one form of corrosion is resistant also to others. Forexample, a nickel-bearing stainless steel may be highly resistant tonitric acid, and yet prone to disastrous cracking when exposed understress to chloride environments.

The alloys of this invention have been developed to resist exposures toa wide variety of corrosive environments, while still having highpost-weld ductility and good economy in the fabrication.

Important types of corrosion include the following:

1. Pitting corrosion in halide environments a. Extreme exposure, as inoxidizing chloride environments, e.g., 10% FeCl .6 H O at 50C.,accentuated by crevices,

b. Severe exposure, as in chloride waters containing permanganate ionsat C.,

2. Intergranular corrosion in acid and chloride environments3.'Stress-corrosion cracking in chloride-containing environments 4.General surface corrosion a. Organic acids, such as sulfamic, formic,acetic, and

oxalic acids,

b. Oxidizing acids, such as 65% nitric,

c. Inorganic reducing acids, such as boiling 10% sulfuric,

(This latter category can best be appraised in three different aspects:

(-1) Active alloys, which are active at once, or. within a few hours,these dissolving at rates in excess of 50,000 mils per year, (II)Passive alloys, which are passive upon immersion in the corrosive media,dissolving relatively uniformly therein at rates less than mils/yr.These alloys become activated when contacted with an activatingelectrode and remain active A. SPECIMEN PREPARATION 1. Ingredients Allspecimens were prepared by the technique hereinafter described, usinghigh purity ingredients as detailed in Table I:

TABLE I Ingredient Supplier Analysis Iron Glidden Co. 99.9l% Fe, C 20ppm,

N 40 ppm Chromium Union Carbide 99.957: Cr, (LUV/l Fe,

Corp. C 50 ppm, N 60 ppm Chromium Shicldalloy Corp. 982% Cr, C 85 ppm,

N 284 ppm Molybdc- Fansteel Co. 99.9% Mo, C 20 ppm,

num

N l ppm Molybdc- Climax Molybdenum 99.771 Mo, C 32 ppm,

num

N 12 ppm Where nickel was utilized, the ribbon form was employed.Silicon was reagent grade, aluminum was in lump form analyzing 99.992%Al, carbon was of High Purity lump grade, free of filler or in the formof high carbon ferro-chrome alloy, and nitrogen was supplied as Cr Npowder.

2. Melting The alloying ingredients were melted in high purity aluminacrucibles in a vacuum induction furnace, which was sealed and evacuatedto 10 to 10 Torr before the power was switched on. The power wasincreased gradually to minimize thermal shock and, when melting wasincipient, the furnace was filled with gettered argon (a purifiedcommercial grade of argon especially low in oxygen and nitrogen content)to an absolute pressure of 5 inches Hg in order to inhibit vaporizationof the alloying ingredients. At the completion of the melting operation,the heat was east through a fire brick funnel into a vertically disposedcylindrical copper mold placed in the argon atmosphere. After cooling,the ingot was removed and the hot top containing the shrinkage cavitywas cut off.

3. Heat Treatment and Working Each ingot was soaked for 3 hours at2200F. in an electric furnace (air atmosphere) and then forged to arectangular cross section.

The forged ingot was then reheated to 2150F. and rolled to a thicknessof 100 mils in light passes, interspersed with four reheats to 2l50F.,each requiring about mins.

4 After the final rolling, the sheet was heated at 2000F. for 1 hour andwater-quenched. Alloys containing titanium as a stabilizing additivewere given a lower final heat treatment of 2 hours at l750F.

Specimens subjected to corrosion, mechanical and analytical tests werecut with a power saw and were thereafter ground to an grit finish usinga watercooled silicon carbide belt.

4. Welding To investigate the effects of welding on corrosion resistanceand on mechanical properties, autogenous welds were made as follows:

Welded samples for bend and stress corrosion tests measuredapproximately 3 inches long X 1 inch wide by 0.1 inch thick, and thesewere given a lengthwise fusion weld using the argon gas-tungsten arcwelding process and an energy input per pass of approximately 16,000joules/inch [the'energy input per pass in joules/inch arc voltage(volts) X are current (amperes)/torch travel speed, in./sec. During thewelding, the back of the sample was concurrently shielded with argon, toreduce oxidization and safeguard against pickup of nitrogen. In furtherexplanation, there was no fusion of two pieces of alloy here, theelectrode simply being given a single pass longitudinally of the samplepiece. During this pass, the energy input was suffieient to melt themetal in the immediate region of the electrode traverse for almost theentire thickness of the sample and for a width of approximately Aiinch.The specimens were then allowed to cool in the air to room temperature,thereby duplicating usual welding practice.

Three specific sample regions are of particular interest in testshereinafter reported, these being the visually apparent weld zone, wherethe torch had melted the surface metal, the remote base-plate zone(abbreviated BP), which is all metal one-half inch or more away from theweld, and the intervening heat-affected zone (HA2).

5. Analyses The data hereinafter reported, and plotted in FIGS. 1 and 2,are based on weighed out proportions of iron, chromium and molybdenum.Confidence in this approach has been provided by a weight balanceestablished by weighing cast ingots and rolled sheets made from theseingots and comparing the results with the total weight of the metalscharged in making the alloys. The average detectable change in weightbetween the weighed-in ingredients, the ingots and the rolled sheetsamounted to only 0.1 gm out of a total weight of 400 gms. Additionalconfidence in the practice arises from the consistency and sharpdefinition of the pitting test results plotted in the FIGURES.

Carbon was determined by combustion with a Leco Carbon Analyzer.Nitrogen analyses were made by the micro Kjeldahl method using NesslersReagent.

Titanium, niobium and aluminum were determined by X-ray fluorescence.

B. ALLOY TESTING l. Pitting Corrosion: Potassium Permanganate-SodiumChloride Test This is a new test applied by applicant to simulatechloride pitting in severely corrosive natural waters, such as OhioRiver water used in heat exchangers. Such waters contain some manganeseand must be chlorinated to prevent the accumulation of organic slime inthe heat exchangers. A propensity towards severe pitting attack results,probably due to the conversion of tetravalent, insoluble manganese tosoluble permanganate (Mn by chlorine and the simultaneous reduction ofchlorine to chloride (Cl) ions.

Service tests at plant locations require relatively large amounts ofmaterial and 6-18 month test exposures for alloy evaluation, so thatthis accelerated test was developed as a substitute. 1

A 2% KMnO -2% NaCl water solution with pH ad justed to 7.5 was employed.Large test tubes 11 /2 inches long X 1 /2 inches dia. containing 150 mlof the test solution were immersed in a 90C. thermostatically controlledwater bath. (The 90C. temperature was selected to simulate conditions inheat exchangers.) The test tubes were covered with a rubber stopperfitted with a glass tube for venting, and the specimens placed thereinwere 1 X 2 X 0.08 inch thick pieces ground to an 80 grit finish.

Pitting attack in the solution is evidenced by extensive formation of asurface coating of insoluble manganese oxides. It appears that, as thealloy dissolves at anodic sites (pits), insoluble manganese oxide isprecipitated at the unpitted cathodic areas where permanganate ions arereduced to the tetravalent state in an electrochemically equivalentreaction.

The coating is removed at room temperature without attack on the metalby immersion of the specimen in a solution disclosed in applicants US.Pat. No. 3,48l,882, consisting of: 900 ml H O, 27.4 ml 96.5% H 30 l4.4goxalic acid, 0.2g Alkanol WXN and 0.2g diorthotolylthiourea. The cleanedspecimen clearly reveals evidence of pitting attack to the unaided eye.

Only specimens which were free of all pitting attack, and of manganeseoxide coating, were classified resistant. Those which displayed anypitting at all were rated failed. Commercially available ferritic andaustenitic stainless steels (e.g., A.l.S.l. 446, 316 and 310) werereadily pitted by this solution at room temperature. Generally,specimens resistant to attack for the first-24 hours were found to beresistant for as long as 16 months.

1n the tests hereinafter reported, samples resistant to this hotpermanganate-chloride test were classified as highly resistant and ofhigh resistance to pitting corrosion.

2. Pitting Corrosion: Ferric Chloride Test This test is commonly usedwhen conducted at room temperature; however, applicant chose toaccelerate it by elevating the test temperature to 50C. and by providingtight crevices. As accelerated, this test is more severe than thepermanganate-chloride pitting test at 90C.

The testwas conducted in a thermostatically controlled water bath at atemperature of 50C. using 150 ml of FeCl 6H O in water in individual I 1/2 X 1 /2 inch dia. test tubes vented through tube-fitted rubberstoppers. The unwelded test specimens, ground to 80 grit finish,measured 1 X 2. X 0.08 inch thick. Crevices were created on the edgesand surfaces of the specimens by employing polytetrafluoroethyleneblocks on the front and back held. in positionby pairs of rubber bandsstretched at 90 to one another in both longitudinal and transversedirections. This created two sharp crevices at top and bottom of thespecimen where the longitudinal elastic touched the metal, two somewhat6 less sharp crevices at the side edges and two crevices under thepolymer blocks. Contraction of the elastics provided constant creviceconditions during progressive metal corrosion at the points of contact.

; At room temperatures, it was found that, if an alloy pits with acrevice it will eventually also pit without a crevice, but the exposurerequired to reveal this may be as long as 4 months duration. lnapplicants accelerated test, pitting occurred within 24 hours in thecase of alloys susceptible to this type of pitting. Resistant alloyswere exposed for weeks, and, in some cases, for as long as 12 months,without any pitting attack.

As hereinafter reported, samples that resisted attack.

in the hot ferric chloride test were classified as extremely resistant.Almost all of the same analyses that passed this test had already passedthe permanganatechloride test.

3. Stress Corrosion: Boiling Magnesium Chloride Test This test, whilenot yet actually adopted as a standard by the American Society ofTesting Materials, is nevertheless already widely utilized. It isconducted in accordance with the procedures described by applicant inassociation with A. J. Sweet, published in Corrosion,

Vol. 25, No. 1, pp. 1-6 (1969) January.

The test solution is boiling (155C.) 45% MgCl The test specimens were 3X A wide, mil thick, in most cases having a lengthwise autogenous weld,because welded specimens reveal susceptibility to stress corrosion morereadily than unwelded specimens. The

welded specimens were bent 180 over a 0.366 inch dia. cylindricalmandrel. Stress was applied by tightening a Hastelloy C bolt throughholes at each end of the specimen, the bolt being electrically insulatedfrom the specimen by polytetrafluoroethylene bushings.

Austenitic stainless steels fail by cracking in 1-4 hours duringexposure to this test. In contrast, it was found that alloys accordingto this invention did not crack within days of exposure. Alloys whichdid not fail sooner were routinely left on test for 100 days todemonstrate their immunity to stress corrosion.

The boiling MgCl test is a very severe one, not usually encountered inindustry. Neverthless, I have found a correlation between it and thestress corrosion propensity of such Crcontaining alloys as AISl-430 and-446 to cracking in NaCl solutions containing only 50 ppm C1. The latteris much more like a simulated service corrosion test; however, testexposures of 250 hours or more are often required to, detect corrosionsusceptibility. Thus, for ferritic alloys, the MgCl test can beconsidered to be a valid, rapid test for evaluating stress corrosioncracking.

Since preparation of welded stress-corrosion crack- 1 ing specimensrequires cold bending welded specimens transversely of the weld, therewas incidentally afforded a severe test of ductility. Some test alloysoutside this invention cracked during bending and were therefore nottested in the MgCl solution. Consolidated test data are set out in theTable I1 hereinafter set forth.

4. lntergranular Attack (lGA): Ferric Sulfate-Sulfuric Acid Test Todetect susceptibility to intergranular attack (hereinafter abbreviatedlGA), welded specimens were exposed for hours to boiling 50% H 80containing 41.6 gm'll Fe2( 4)3 X H O. This rapid test was originallydeveloped by applicant for austenitic stainless steels (M. A. Streicher,ASTM Bulletin No. 229, pg. 77 (1958) April, and ASTM-A262-68 RecommendedPractice for Detecting Susceptibility to lntergranular Attack inStainless Steels). Applieants extensive investigation has nowestablished that this test is also valid for the determination ofsusceptibility to IGA in commercial ferritic stainless steels of theclass represented by AISI-430, -446 and of this invention, as a functionof heat treatment and Cr, C and N contents.

The test was conducted on specimens ground to 80 grit finish, measuringabout 1 X 2 X 0.08 inch thick with an autogenous weld across the widthof the specimens. The specimens were immersed in 600 ml of test solutionheld in a 1 liter Erlenmeyer flask fitted with an Allihn condenser.

Specimens tested were evaluated by both weight-loss measurements and,especially, by 80 X microscopic examination for evidence of graindropping. Three zones were particularly examined for dislodged grains,the base plate (BP), the weld metal (Weld) and the heat-affected zone(HAZ). Any evidence of dislodged grains was cause for rejection of theparticular alloy sample. The results are tabulated in Table II.

5. General Corrosion in Acids As hereinafter set out in Table III, acomparison was made of commercial alloys with alloys within the limitsof this invention as regards general corrosion occurring inrepresentative acid environments, including oxidizing, reducing, organicand inorganic. The acids, techniques and data for commercial alloys havebeen previously published by applicant in Corrosion, Vol. 14, No. 2, p.59t-70t, February (1958).

Briefly, all tests were conducted on unwelded specimens measuring l X 2inches about 80 mils thick, with surfaces ground to an 80-grit finish.Boiling test solutions of 600 ml volume were employed using Erlenmeyerflasks fitted with reflux condensers. Tests showing astronomica1"corrosion rates lasted only 5 minutes, but for samples corroding at lessthan 100 mils/- year, the tests were prolonged for 100 hours.

Especially significant, as detailed later, is a group of tests utilizedto show the development and/or loss of passivity, and the corrosion ratein boiling sulfuric acid.

6. Mechanical Tests In addition to the bend tests made preliminary tothe MgCl stress corrosion test of Section B( 3) supra, a number ofadditional mechanical tests were made to obtain a comparison withcommercial steels of the same general class and, in any case, toestablish critical strength data.

Thus, a tensile test was conducted on alloy Q-202-H made according tothis invention, the analysis of which was 28.5% Cr, 4.0% Mo, C, 23 ppm,N, 130 ppm. The results, as compared with commercial steels havingproperties tabulated in the Stainless Steel Handbook published by theAllegheny Ludlum Steel Corp., pp. 2-5 (1951) were as follows:

Alloy 75F. -25F. 50F.

O-433 bent hent bent cracked [Cr 28.5%, M0. 4.0%

C18 ppm, N 37 ppm] Q-436 bent bent ICr 28.0%, M0. 4.0%

C 28 ppm, N 83 ppm] Q-437 bent cracked {Cr 27.5%, M0 4.0% C 29 ppm, N 65ppm] Yet another mechanical test was a cold rolling test in which thefollowing alloys of this invention, which had previously been hot-rolledto a thickness of about mils, were cold-rolled to about 25 mils, thelimit of the rolls:

Per Cent Alloy Cr(%) Mo(%) C(ppm) N(ppm) Reduction Q-l 20 30.0 3.0 90Q-ZOZA 28.5 4.0 20 25 81 Q-562 35.0 3.5 14 20 69 Q-557 33.0 4.5 28 35 70Q-514 30.5 4.0 5 I70 67 In every case, there was excellent ductility,i.e., there was no cracking, either at the edges or in the surfaces.

In still another investigation, comparative Charpy impact tests were runon a 29.0% Cr, 4.3% Mo, 25 ppm C, ppm N specimen according to thisinvention, labeled Invention in the tabulation infra, along withAISI-446 and -316 commercial steels.

All Charpy specimens were half-size, i.e., 2.16 X 0.197 X 0.394 inch,with a 45 notch having a 0.010 inch radius. These specimens weremachined from inch thick plates with the root of the notch lying in therolling direction.

Type

Alloy of Fracture AlSl-446 Charpy Impact (ft-lb.)

AISI-3l6 42.75, 47.5 45.0

lnvention 44, 5]

From the foregoing, the Charpy impact values for alloys of thisinvention were about the same as for AISI-3 16 and much superior tothose of AISI-446.

C. EVALUATION OF Fe-Cr-Mo ALLOYS LIMITED IN C AND N CONTENTS BUTCONTAINING NO OTHER Additives Beyond Incidental Impurities Referring toFIG. 1, a great number of alloy composi- 9 tions are plotted whichcollectively precisely define a number of different regions A, and A,(which can, for some purposes, be considered together to be an entityA), B, C, and C (which can, for some purposes, be considered together tobe an entity C) and D according to this invention which arecharacterized by improved corrosion resistance, especially pittingresistance, over the prior art. In addition, these several regions arecharacterized by different corrosion resistances among themselves,generally showing increasing corrosion immunity with increase in both Crand Mo contents within the overall perimeter enclosing all of theregions.

The vertical division line at 27.5% Cr defining the areas made up ofregions A, and C, to the left and A and C to the right can bedisregarded in the general consideration of corrosion resistance as towhich Table ll pertains; however, this dividing line has significanceTABLE ll a. Regions A, and A collectively, characterized by resistanceto pitting under exposure to (l) the permanganate-chloride test and (2)the ferric chloride test, (3)

15 resistant'to intergranular corrosion attack [lGA] under exposure tothe ferric sulfate-sulfuric acid test, (4) ductile in the 180 transverseweld bend test of asreceived (unannealed) welded specimens and (5)resistant to-stress corrosion [S.C.].

Composition in Per Cent by Wt.

Alloy Cr and Mo, ppm

No. I C and N Remarks Region A, Cr Mo C N 665 1 25.0 5.5 75 150 Nottested for stress corrosion 438 27.0 4.0 24 68 Passed all 5 tests 57725:5 5.5 r 25 63 Test 3 [IGA] omitted 549 g 27.5 5.5 195 Passed all 5tests 548 v 27.5 5.0 10 5 Tests Nos. 1 & 3 [lGA] omitted I 496 27.5 4.531 155 .489 26.0 5.5 19 I 108 Test No. l (KMn0,NaCl) omitted 488 26.05.0 22 1 l0 Passed all 5 tests Composition in Per Cent by Wt. Alloy Crand Mo, ppm

No. C and N Remarks Within Region A, Cr Mo C N 656 28.5 4.0 23 100 TestsNo. 2 and No. 5 for FeCl;

and stress corrosion, respectively, omitted 611 29.5 4.7 25 118 TestsNo. 3 [lGALand No. 5

[S.C.] omitted 610 28.5 3.5 25 Tests No. 1, No. 3 and No. 5 omitted 58528.5 4.5 20 93 Passed all 5 tests 559 30.0 4.0 24 I50 Tests No. 3 [10A]and No. 5

' [S.C.] omitted 554 28.5 4.2 23 17 Tests No. 3 [16A] and No. 5

i [S.C.] omitted 548 27.5 5.0 10 5 Tests No. 1 and No. 3 [lGA] omitted547 27.5 3.8 15 Tests No. 3-5 omitted 544 29.5 3.2 24 1'18 Tests No. 3[IGA] and No. 5

[S.C.] omitted 543 29.0 4.7 27- 13 Test No. l KMnO,NaCl omitted 54] 29.54.5 38 Tests No. l-3, incl., omitted 539A 30.0 3.5 15 I 128 Test No. 3[lGA] omitted 538 28.5 4.5 29 15 Passed all 5 tests 537 28.5 4.5 23 133518 31.0 v 4.0 21 88 Tests No. l and No. 3 [16A] omitted 517 31.0 3.0 14188 Test No. 3 [10A] omitted 513 30.0 4.5 19 150 Tests No. l and No. 3[lGA] omitted 436 28.0 4.0 28 v 83 Passed all 5 tests and, in addition,was ductile at 75 F.

Composition in Per Cent by Wt. Alloy Cr and Mo, ppm

No. C and N Remarks Peripheral Cr Mo C N Analyses Outside Regions A, andA, (Underscorcd Alloy Nos. plotted on FIG. 2)

. .-cont1nued Composition in I L 2.: 1 Per Cent b'yawt. 1'

h Alloy A Cr and Mo,-ppm j p No. i. I y C and N i Remarks. 'Peripheral'Cr" Mo C' i v N i Analyses;

fiutside Regions i g i Tcst N o. 4 (Bend).

- Tests No. l. 3 and 5 omitted 494 27.0 .0 10 305 Failed Tcst No.4tBendl.

' Tests No. l and 5 omitted 502 28.0 6.0 9 165 504w 2x5 5.5 10 m FaildTcstNo. s (so g I i Test No. L- omitted x 51 l v 29.5 5.0 ll l' ailcdTest No. 4 (bend).

'Tests' No." I. No. 3 & No. 5 rr- 1 omitt'ed- 48l 29.5 4.8 9,3 $8 FailedTestNo. 5 (8.0. L g

; Test Noni omitted I I I Y i 558 33.0 5.0 22 '5" Failed T est Nb. 4(Bend). I i

' Tests No. 3 84 No.5 omitted M6 35.0 5.0 20 ;203 Failed TestNo. 4(Bend). 1'

' i Test'No'. 5 omittcd 603 35.0 4.5 l I 1 l5 Failed Test No. 4 (Bend).

Tests No. 3 and No. 5 omitted c. Region B, characterized by resistanceto pitting under exposure to (l) permanganate-chloride test and (2)ferric chloride test, (3) resistant to intergranular corrosion attack(lGA) under exposure to the ferric 0 sulfate-sulfuric acid test, (4)ductile in the 180 transverse weld bend test of as-received (unannealed)welded specimens and (5) resistant to stress corrosion (S.C.). Inaddition, all region B and D specimens are passive in boiling H 50 ashereinafter set out in Table IV; however, region D specimens otherwisehave the properties of regions C and C i.e., they fail the ferricchloride Test No. 2.

b. Regions C and C collectively, characterized'by resistance to pittingunder exposure to (l) permanganate-chloride test, (3) resistance "tointergranular co rrosion attack (lGA) under exposure to ferricsulfatesulfuric acid test,\(4) ductileuin the 180 transverse weld bendtest of as-receivcd (unannealed) welded specimens and (5) possessedofstress-corrosion resis-T tance to extent tested. The following specimensall failed Test No. 2, the ferric chloride pitting test.

Composition in Per Cent by Wt. Alloy Cr and Mo. ppm I No. C and NRemarks Regions C and C (except Alloy No. 568. which is just below) Cr 0C N I 625 27.0 4.0 l5 190 Passed Tests No. l. 3 and 4 -Not tested forSC. (No. 51- 624 26.0 3.5 1.7 .150 576 23.0 6.0 6 43 .Test No. 3 lGAomitted.

; Passed S.C. test 57l 26.5 3.0 10 l IS in addition to Test No. 2'. TestNo. l (KMnO NaCl) alone run g (and passed) 568 Y 27.0 2.5 5 120 FailedTest No. l. Tests No. 3

- and No. 5 omitted 567 25.5 4.0 5 75 In addition to Test No. 2. TestNo. l (KMnO NaCl) alone run (and passed) 666 22.0 I 6.0 l 52 ll0 PassedTests No. l. 3 8L '4.

' Not tested for SC. 597 30.0 570 28.0 13 -98 In addition to Test No. 2.Test No. l (KMnO.,-NaCl) alone run 1 (and passed r 520 32.0 2.0 I7 50Passed Tests No. I. 3 8L 4.

- Not tested for S.C.

Mid

Slfi 31.0 2.5 7 175 508 29.5 3.0 15 163 Tests No.2. 'No. 3 84 No. 4alone run.

Failed No. 2 and No; 3 (.IGA) 457 29.0 3.0 I28 Tests Nofl. No. 2 8L No.3 alone run.

Failed No. 2. passed No. 1 {it No. 3 503 28.5 3.4 5 160 Tests No. 2. No.'3 and No. 4 alone 1' run. Passed No. 3 and No.4 I 435 29.0 3.0 46PassedTests No. 1.3.4 & 5.

1 failed No. 2 v 1 Composition in,- Per Cent by Wt.

- Passed all tests. t v Passed all 5 tests Passed Tests No. l-4. incl.Test No. 5 (S.C.) omitted I V, i I P Passed'l'ests No. lb 2,4 and "5;Test No. 3 (lGA) omitted v 555 33.0 3.0 48 23 52| 32.0 4.0 I5 45 PassedTests No. 2, 4 & 5.

Tests No. l and ,N o."3, (lGAl omitted Region D 560 33.0 2.0 1 I6 85Passed Tests No. l, 3 and 4.

|.. No. 5 (S.C.) omitted;

As hereinbefore mentioned in- Section 8(5), compar- The following tests,reported in Table IV, illustrate atlve general corrosion resistance totypical common the critical compositional relationship necessary to acidenvironments, including oxidizing, reducing, orachieve the highresistance to boiling 'l0% sulfuric acid game and inorganic acids, isset out in the following corrosion possessed by alloys lying withinregions B and Table III: -D, FIG. 1. 1

TABLE III COMPARISON OF GENERAL CORROSION OF ALLOYS lN ,AClDS* GeneralCorrosion (Boiling) (mils per year) 507: Sulfuric 1 with Ferric SodiumSulfuric Alloy Nitric Sulfate Sulfamic Formic Acetic Oxalic BisulfateAcid 657: 10% 45% l0%. l0% 10% Y AlSl 430 20 i 312 l44,000 84,700 3,0006,400 9l,200 252,000 AlSl 446 8 36 150,000 9,700 0 7,000 64,800 270,000AlSl 304 8 23 l,300 1,715 300 570 2,760 l6,420 AlSl 316 ll 25 75 520 296 I70 855 Carpenter 20 8 9 7 l6 7 2 7 ll 43 Hustelloy C 450 240 8 5 0 I8 i 8 17 Titanium 1 140 285 1873 0 950 I 250 6,290 Fe-Zli'k Cr-4"/ Mo(1) 2 .l 6 0 h l 0 l3 9 '52,l80 l e-33% Cr-3% Mo (2) f v 60 (1) Alloy O202. having C 23 ppm, N l ppm (2) Alloy O 555. having C 48 ppm. N 23 ppmAcid concentrations-in per cent by weight TABLE IV CORROSION OF Fe-Cr-Mo Aeeovs mnoluno l0% SULFURIC A'clb' v Composition I I Corrosion PerCent by Wt. ppm v State (I) Rate (2) Alloy No. Cr Mn C N (mils/yr.)

513 30.0 4.5 9 150 active 44,200 539-A 30.0 3.5 l5 l28 active l95,200

6l 2 (FIG. 2) 3l.0 5.0 25 290 .active 48,000

51'9 I 31.0 4.5 l8 100 7 active 53.200 518. 3L0 4.0 2| 88 I 5 active62,500

627 510,2). 31.0 5.5 l0 265 active 72,100

628 (FIG. -2) 3l.5 3.0 7 235 active 83,400

52l 1 32.0 4.0 I l5 passive 4 v .75 .629 32.0 3.0 l6 H passive I 1 4 5659 32.0 2.75 '45 140 passive A '58) (FIG Z) 32.0" 2.5 22 215 passive 55520 ii; 32.0 2-,0 l7 50 active 11.6.000

- 32.0 M 0.0. 25 l 70 active 54,000 33.0 '4.5 i 28 '35 passive 70 33.04.0 25 53 passive I, 65

v v 33.0 300 48 23 passive 60 I 33.0 i 2.5 46 98 passive 50 33:0 2.0" 16passive 45 33.0 1.5 22 passive 40 35.0 v 39 320 passive 50 -Paxsivi.- n6sihle evolution of hydrogen, low'c'orrosion rate. (2) Rate inactivealloys de'termiped in 5-r n in .-te st. Rates on passive alloysdetermined in l00-hr. test.

The following Table V lists the analyses and test results for a largenumber of Fe-Cr-Mo alloys which do not meet the compositional limits ofthis invention, particularly as regards C and N contents. These Alloyrespects. For example, below region C the alloys suffer both seriouspitting corrosion in the less severe Test No. 1 (permanganate-chlorideexposure) and may also be subject to intergranular attack, withresultant grain Nos. are plotted w1th1n the overlay of HO. 2, and thedropping, although they may be ductile after weldlng. several causes oftest failure are denoted by character- Below region D, the alloys suffernot only pitting 1st1c point symbols defined 1n the drawtng legend. Fromcorrosion and intergranular attack but are also brittle Table V, takenin conjunction with FIG. 2, it can be after welding. To the right ofregions B and D, the seen that the contents of both C and N are sharplyalloys are brittle after welding, whereas, above area A cr1t1cal, andthat this criticality is also affected, to some and region B, the alloysare either brittle, so that they degree, by the associated Cr and Mo.break during bending after welding, or otherwise they TABLE V FIG. 2DATA TEST RESULTS STRESS COMPOSITIONS lN KMnO,- CORRO- ALLOY wT. PERCENT Cr 82 M0, NaCl FeCl Fe.,1SO.. BEND 510M NO. PPM c AND N Test No. 1Test NO. 2 H. .so Test NO. 3 TEST NO. 4 TEST NO. 5

Cr M0 C N HAZ WELD BP 529 27.5 4.2 16 208 P P P P P P P "532 28.5 4.5 24353 P P F P P P P 627 31.0 3.5 10 265 P P P P P P P 668 35.0 4.0 39 320P P P P P P 493 27.0 5.5 223 P P P P P P 453 29.0 4.0 18 239 P P P P P P492 27.0 5.0 10 283 P P P P P. F 628 31.5 3.0 7 235 P P F P P P(F)* 61231.0 5.0 25 290 P P P P P F 615 35.0 2.5 23 100 P F P P P F 630 35.0 3.57 185 P P P P P F 657 28.5 4.0 56 198 P P P P P P 458 28.5 4.0 114 208 PF F P P F 459 28.5 4.0 118 F P F F P F 599 33.0 3.0 109 68 P F P F P P P494 27.0 6.0 10 305 P P P P F 613 34.0 2.0 26 300 P F P P P F 497 28.03.5 29 209 F F P P P 594 25.0 5.0 18 268 P F F P P F 463 28.5 4.0 14 239F P P F F 4098 29.0 4.7 856 219 P P F F P F 450 27.5 3.0 14 204 P F P FF 452 28.5 3.0 33 267 P F F F P 460 28.5 4.0 171 P F F F F P F 464 28.54.0 22 239 P F F P P F 487 26.0 1.0 26 204 F F F F P P 589 32.0 2.5 22215 P F F F P F "*531 28.5 4.5 .334 25 P P F F F F 461 28.5 4.0 189 89 PF F F F F 582 27.0 3.0 48 255 F F F P P P P 587 33.0 1.5 22 195 P F P PP F 530 26.0 1.0 15 90 F F F F P P P 408 29.0 4.7 48 372 F F F F SecondSample "Deficiency cured by heat I hr. at 2(10UF. and water quenching.Deficiency not cured by heating l hr. at 20U0F. and water quenching,

not tested {P Passed F Failed D. SUMMARY From the foregoing, it will beseen that the alloys of my invention have post-welding ductility andgood stress corrosion resistance besides being,

1. in area A, made up of regions A, and A collectively, extremelyresistant to pitting corrosion as regards both Tests No. l,permanganate-chloride, and No. 2, ferric-chloride,

2. ln area C, made up of regions C and C collectively, highly resistantto pitting corrosion as regards Test No. l,

3. ln region B, equally resistant as area A, plus passive and resistantto corrosion in boiling 10% H 4. In region D, equally resistant as areaC, plus passive and resistant to corrosion in boiling 10% H 80 Outsideof areas A and C and regions B and C, taken together, Fe-Cr-Mo alloysare deficient in one or more E. ADDITION OF OTHER METALS TO Fe-Cr-MoALLOYS In order to determine possible benefits of other additives, anumber of specimens were made up containing 28-29% Cr, 4-4.5% Mo, plussingle metals in the ranges set forth in Table VI. The specific purposesfor which the several additions were made are indicated, together with abrief report of side effects noted.

TABLE VI Component Achievement of Purpose and Amounts Purpose OtherEffects Altlrnii'ium Grain refitiel' Yes '-0.l0 0.60% g Titanium or a)To prevent [GA a) No. l.G.A. above inven- Nio'bium 3 5 tion's specifiedC, N 0.20 0.6054 b) Grain refi'rier limits. Bend cracking tendencyincreased. I h) Yes. Grain was refined. Plu'tihtim Field A- Cz 0.0060.30% passivity in Yes. Continued region A- boiling 1072 C properties.

I H 50 Ptllludium Passivity in Yes. Lost pitting rc- (l.02 0.2071hoiling I071. sistance in both H- SO Tests No. l and No. 2. iridium Yes.Continued region 0.015 0.10% A C properties. Rhodium Yes. Resistant inTest No. I 0.005 0.l0'/r but not in Test No. 2.

One sample. near the N limit of 200 ppm? showed l.G.A. Osmium Yes.Osmium oxide has 0.02 0.l0 /i high vapor pressure and is toxic.Continued A C properties. Ruthenium Yes. No deleterious effects 0.020L507: observed up to 0.30%

Ru. Suffered stress corrosion above 0.30% level. 0.02% Ruthenium Yes. Nodeleterious 0.307! Aluminum effects observed.

' Grain refinement I noted.

0.01% Ruthenium Yes. Region A require- 0.20% Nickel merits met, and nostress corrosion on welded specimen despite Ni. I 0.20% Gold Yes.Resistant in Test No. l.

but not Test No. 2. Nickel 0.25 to Yes. Stress corrosion 2.071resistance progressively dei creases as nickel content increases; Nickel2.0 3.0'7: Yes. Sc|f-repassivating.

' and resistant in Test No. l. but not Test No. 2. Cobalt 2.0 4.0% Yes.Stress corrosion re sistance seriously decreased. Not rcsistant in TestNo. 2. Addition of sili- Mo re- Yes. Resistant in Tests con in range l.5placement No. l and No.2. 2.071 to alloys Containing 27 3071 Cr and 1.52.0%

Mo. 0.80% Mn 0.5071 Commonly Yes. No harm done to Si 7 present in a anyRegion A:

commercial properties. heats. 0.20% Cu or Commonly present Yes. No harmdone to 0.1571 Ni. singly, in commercial Region A or 010% Cu heatsproperties.

tivein producing the d For the additions of ruthenium and nickel; respecesired results.

'tively, the entries of Table V! are expanded as Tables Vll and VIII,where the individual results'for several samples are shown. In addition,these Tables show the self-repassivating effect obtained ,whensufficient of TABLE v11 either additive, Ru or Ni, respectively, ispresent.

EFFECT OF RUTHENIUM ADDITIONS TO Fe 28% Cr 4 4% Mo Al .LOY

Behavior in Ruthenium Boiling l lll H 80, Stress Alloy AdditionCorrosion Rate Pitting Corrosion v Corrosion (3) No. ('7: by Weight)State (mils/year) KMnO.,NaCl (l) FeCl;; (2) (Boiling 45% MgCL) 338'0.015 active 62,200 477-/\ 0.017 active P 334 0.020 passive 60 P PResistant (not welded) 542 0.20 passive 9 P TABLE VII-continued EFFECTOF RUTHENIUM ADDITIONS To Fe '28%-Cr 47; Mo ALLOY Behavior in RutheniumBoiling I07: H 80 Stress Alloy Addition Corrosion Rate Pitting CorrosionCorrosion (3) No. (/1 by Weight) State (mils/year) KMnO..NaCl (I) FeCl;(2) (Boiling 45% MgCl 475 0.30 passive 2 Resistant (welded) 683 (1.50passive* 7 Failed (welded) 671 0.75 passive" 2 Failed (welded) 684 l.50passive" 2 Failed (welded) plus 0.20 Ni passive 40 1 P I P Resistant(welded) self-repassivating (112% KMnO. 2% NaCl at 10"C. (2) 1071 FcCl oH O at 50C. with crevices.

(3) Magnesium chloride test.

P No pitting Not tested TABLE VIII. 1 1

EFFECT OF NICKEL ADDITIONS TO Fe 28% Cr 4% 'Mo ALLOY Behavior in NickelBoiling 10% H SO. Alloy Addition. Corrosion Rate Fitting CorrosionStress No. by Weight) State (mils/year) KMnO NaClfi FeCl;,(2,)Corrosion(3) 436 0.00 active 52,000 -P" P Resistant (welded) 677 0.10active 63000 P P Resistant (welded) 239 0.20 active Pf P Resistant 2170.25 passive 56 P P Failed (welded) 183 0.30 passive 52 P P Failed afteri 119 hours 191 0.40 passive 29 P I P Failed after I 26l hours 241 0.50passive 24 P i P Failed after i 16 hours 245 1.50 passive 6 P l P Failedin less than 16 hours 245 1.50 passive 6 P Pv Failed in less 1 4 than 16hours 681 1.80 passive l l P. 1 P 664 2.00 passive* X P P 658 2.50passive* 10 P I F 649 3.00 passive* 9 P F Footnotes for Table Vlll(1)271 KMnO -2'7( NaCl at 90C. 1 (2)1054 FcCl;..o,H O at 50C. withcrevices. I (3)Magnesium chloride test on unwelded specimens except asnoted. P Passed. no pitting F Failed. pitted Not tested These alloys arealso self-repassivating.

The effectiveness of nickel in conferring passivity in of demarcationsetting off area A from A andC from H 50 is a function of both chromiumand molybde- C in FIG. 1. num, as shown in TABLE IX. Thus, positivebenefits 50 H In addition, as indicated by Alloy No. 634 in TABLE accrueabove a molybdenum content of about 2.0%- lX,i,alloy's'containing thespecified minimum of rutheand with the approximate lower essential limitfor chroniumappear to require the same 27.5% minimum chromium 27.5%,thereby locating the broken vertical line Y mium.

TABLE 1x I i NICKEL RUTHENIUM ADDITIQNS IO. F1:-C r-Mo ALLOXS Boiling1071 Compt'isit ionil 'i Sulfuric Acid i Pitting'Corrosion l Stress 1Alloy Cr 1 Mo Nickel 2 a State KMnO 2NaCl(2) .FeC.l,('3) CorrosionM) 1-15. r (not welded) 1 0-231 25.0 410- 0.40 active F F 'F'ailed'after447hrs.

0-232 26.0 4.0 0.40 h v active P F Resistant 0-233 27.0 4.0 0.40" 1active P F Failed after 447 hrs.

0-191 28.0 i 4.0 "0.40 passive i P" P Failed after 261 hrs. Q-l96 28.50.0 0.40 u iv e F F 0195 211.5 Y 1.0 h 0.40 active F I F .0194 zx s 3.0?0.40 I passive F 0.193. 221.5 .tlo 0.40 passive P F 0-192 28.5 3.5 0.40passive P P TABLE IX-continued EFFECT OF NICKEL AND RUTHENIUM ADDITIONSTO Fe-Cr-Mo ALLOYS Boiling Composition( 1 Sulfuric Acid PittingCorrosion Stress Alloy Cr Mo Nickel State KMnO,-NaCl(2) FeCl;,(3)Corrosion(4) (not welded) Ruthenium O-634 26.0 1.0 0.02 active F F (HPercent by weight,

(2)271 KMnO, 2% NaCl at 90C.

(3}[0'71 FeCl h H O at 50C. with crevices. (4)Magncsium chloride test onunweldcd specimen. P resistant F pitted The research on additives ofTable VI indicates that:

1. Aluminum can be added up to 0.60% to the compositions of thisinvention in order to obtain grain refmement.

2. Titanium and niobium, in contrast with the opposite expectation basedon prior art, were not effective in my Fe-Cr-Mo-containing alloys to fixexcessive C or N, although they did produce a grain refinement similarto that obtained with Al.

3. The noble metals aided region A compositions to achieve passivity inboiling 10% H 80 but palladium especially, and rhodium to a lesserdegree, reduced the pitting corrosion resistance. Of the noble metals,ruthenium is especially attractive because of moderate cost,effectiveness in small amounts, and freedom from loss in pittingcorrosion resistance.

4. Nickel is effective in producing passivation, but the quantitiesrequired make the alloys prone to stress corrosion cracking in MgClsolution. However, 0.01% Ru 0.20% Ni provided passivation without lossof stress corrosion resistance.

5. Nickel in the range of 2.0-3.0% causes the alloy to acquire theproperty of self-repassivation (refer Table VIII). There is, however,accompanying loss in pitting resistance in the ferric chloride test, andin the magnesium chloride stress corrosion test.

6. In alloys containing 2730% Cr and 15-20% Mo minima, it is feasible toobtain enhanced corrosion resistance (i.e., the properties of Region Aby additions of 1.5-2.0% Si.

What is claimed is:

1. molybdenum alloy having good post-welding ductility consistingessentially of chromium and molybdenum in the weight percentages withinareas A and C of FIG. 1, 0.25 -3.0 weight per cent Ni, carbon ppmmaximum, nitrogen 200 ppm maximum, and carbon plus nitrogen 250 ppmmaximum, the balance being iron and incidental impurities.

2. A corrosion-resistant ferritic iron-chromiummolybdenum alloyaccording to claim 1 incorporating 28.530.5% Cr and 3.54.5% Mo.

A corrosion-resistant ferritic iron-chromium IJNHEE STATES PATENT GFFEQEQER'EEHQCAEE @F QQRREQ'HGN Q1 PATENT NO. 3,929,475

DATED December 30 1975 INVENTOR(S) Michael A Streicher It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown beiow:

Front page, under "RELATED U58. APPLICATION DATA", after March 9 1971,delete "abandoned".,

Column 6, line 2'? before widefl, insert --inch--. 6

7 Column ll in Table at top of page, under "Alloy E10,", the

numerals 49a and 616 should be underscored.

Column l t, Table IV, first line, the "0" value for Alloy No, 513 shouldbe -=-19-- instead of "9". 0

gigned and Sealed this twenty-seventh Day 0f April 1976 [SEAL] Arrest:

RUTH c. MASON c. MARSHALL DANN a Arresting Officer Commissioneroflalentsand Trademarks

1. A CORROSION-RESISTANT FERRITIC IRON-CHROMIUM-MOLYBDENUM ALLOY HAVINGGOOD POST-WELDING DUCTILITY CONSISTING ESSENTIALLY OF CHROMIUM ANDMOLYBDENUM IN THE WEIGHT PERCENTAGES WITHIN AREAS A2 AND C2 OF FIG. 1,0.25-3.0 WEIGHT PER CENT NI, CARBON 100 PPM MAXIMUM, NITROGEN 200 PPM 2.A corrosion-resistant ferritic iron-chromium-molybdenum alloy accordingto claim 1 incorporating 28.5-30.5% Cr and 3.5-4.5% Mo.